Move the WebDAV page to a "Code" subspace and migrate the translation keys

Currently the WebDAV UI is located in XWiki.WebDAV.  I guess it will be better located in another place, especially as it still uses translation keys with the prefix xe.webdav., and the corresponding translations are still in the ApplicationResources.properties in oldcore.

Looking at the best practices I guess this should be moved to WebDAV.Code.UI (also also add a new page WebDAV.Code.Translations, and move the translations to use new keys with prefix org.xwiki.contrib.webdav.. (Or should I just use webdav. as prefix? This seems to fit with the translation best practices but looks prone to naming collisions to me ...)